A group of 850 homosexual men at high risk for AIDS has been characterized in a comprehensive effort to study the epidemiology of this condition. The majority of these volunteers have participated in previous studies conducted by our laboratories over the past decade. As a result, in the first 18 months of this project it has been possible to define: 1. The time of anti-HTLV-III seroconversion for most antibody positive men during the past eight years; 2. A relationship between the duration of seropositivity and the degree of cell mediated immune (CMI) alterations and the risk of developing AIDS; 3. Alterations of CMI indices at the time of seroconversion which differ from those seen after long-term seropositivity. The objectives for the next five years are: 1. To identify viral and host factors related to disease progression; 2. To prospectively determine whether the relationship between the time of onset of AIDS virus infection (antibody positivity) and the appearance of defective CMI or AIDS is related to the incubation period alone or at least in part to possible cohort effects; 3. To investigate the existence and duration of AIDS virus positivity in anti-HTLV-III negative individuals; 4. To evaluate infectivity and susceptibility to the AIDS virus in virus positive/negative paris of sexually active partners; 5. To evaluate prospectively the ongoing risk of AIDS virus infection among susceptible homosexually active men; and 6. To maintain and expand a repository of sera and mononuclear cells for future testing as indicated, especially for tests related to the above objectives. These objectives require an integrated epidemiologic application of immunologic, immunogenetic, virologic and molecular methodologies.